This invention relates to an electrocardiogram playback system for obtaining an accurate electrocardiogram waveform, particularly one with less time-base fluctuation.
Holter recorder systems are conventionally in use for recording and playing back electrocardiograms. In that system, a portable electrocardiogram recorder/playback unit is used to obtain analytical electrocardiogram waveforms by recording electrocardiogram signals on a magnetic tape for hours at low speed and later playing back the tape at high speed. Although Holter systems are convenient, time-base fluctuation in Holter systems must be held to about 3% or less to accurately analyze the reproduced electrocardiogram waveform.
The playback signal time-base fluctuation often becomes large, however, when the low-speed recording alone is simply played back at high speed. In consequence, a reference clock signal is simulataneously recorded on another track in the conventional Holter recorder system while an electrocardiogram signal is being recorded. The reference clock signal is then used to servo-control a capstan-rotating mechanism of the playback unit when the electrocardiogram is reproduced. Notwithstanding, the playback signal time-base fluctuation remains in the neighborhood of 10%, which is still too high for accurate analysis of an electrocardiogram waveform.
In video technology time-base collectors are used for correcting playback signal time-base fluctuations. The time-base collector corrects the video signal time base (jitter) by extracting a clock signal with a horizontal or vertical synchronizing signal as a time reference. The time-base collector is indeed an ingenious device utilizing the special characteristics of a video signal, namely a discontinuous waveform in which a synchronizing signal can be inserted.
Since the electrocardiogram signal is a continuous analog signal, however, it is difficult to insert a signal equivalent to the synchronizing signal into the electrocardiogram signal, and therefore a time-base collector cannot be easily employed for correcting the time-base fluctuation in the electrocardiogram playback waveform.
As another possibility for correcting the playback signal time-base fluctuation, the continuous electrocardiogram signal could be divided at proper intervals and compressed in terms of a time-base. A synchronizing signal can then be inserted into the time gap resulting from compression, and recording is then carried out. At the time of playback, the time base is corrected with the synchronizing signal as a reference signal. However, this time-base compression method necessitates more circuitry which precludes use in an electrocardiogram signal system employing a compact, lightweight recorder like the Holter recorder.
Another way of dealing with the playback signal time-base fluctuation, is found in the field of digital recording technology, such as CD (Compact Disk), DAT (Digital Audio Tape). Time-base fluctuation in such a transmission system is minimized by converting an input signal into a digital signal and transmitting the digital signal to a playback unit. This method, as in the case of the above mentioned time-base compression method, also causes the device to become too complex and large to be used in electrocardiogram signal playback